Question: What is a Mary Sue/Gary Stu?

Answer: It is a OC (Original Character) with over idealized and too
familiar mannerisms and characteristics, lacking flaws, implausible talents and most often acting
as wish-fulfillment fantasies for their creators. Some even representing their creators in a way, this is known as a 'Self-Insert'

Or to put it more simple, a character that is too perfect with no weaknesses and basically a disguised
wannabe of the creators themselves quite commonly attempting to make
canons they are fond for fall head over heels for them.
The creator of a Mary Sue or Gary Stu (male version) will often push their audience into showing how
wonderful or glorious the character is which often leads in people having a great dislike for the character.

Here are some common signs of a Mary Sue/Gary Stu:

-Ridiculously unrealistic
-Everything goes their way.
-Never seems to fail, lose or do wrong
-In many cases the creator attempts to make Canons or other OCs of interest fall for them.
-Possessing extreme or exotic skills, weapons and abilities
-Often having an unusually tragic past
-In RP, frequently or occasionally godmodding
-Being shown as 'special', 'amazing' or 'unique' amongst the character's race or species
-Young, usually adolescent or somewhere in early 20s
NOTE: Not saying every single one is a Sue/Stu, this is just the common age choice amongst creators as Sues/Stus in this age range would be an excuse for them to be extremely attractive, at it's physical prime and/or with racing hormones
-Overly cute, strong, egotistical or flirtatious to the point where is is annoying to other RPers
...aaaand, other RPers tend to keep their characters away from them and, at times, try to kill or
show constant dislike towards them.

Overall...they are poorly planned characters.
Take your time and plan your characters carefully, make other people enjoy them as much as you do.
Don't be afraid to give your characters some weaknesses (nobody's perfect of course) and make sure they're suitable for whatever universe you wish to include them in.

Showing Mary Sue/Gary Stu characteristics in character sheets will most likely gain disapprovals
So obviously to prevent this, avoid showing the signs shown above in your character sheet(s) as well as in RP
We do not accept Mary Sues or Gary Stus as they annoy most other RPers,
cause drama and they are painful to come across in RP